A conventional receiver operates over a specific frequency band, which is called the passband. The receiver components have a limited dynamic range over which they operate linearly. If the signals in the receiver are large enough to exceed this dynamic range then the receiver exhibits some non-linear behaviour. This behavior includes mixing between different signals, and self-mixing of each large signal.
Mixing between different signals results in intermodulation distortion (IMD), while self mixing of a signal results in harmonic distortion (HD). The IMD and HD can fall directly into the passband, which degrades the signal to noise ratio (SNR), since this distortion appears as noise in the desired signal.
A duplexer or bandpass filter is placed at the input to the receiver to attenuate signals outside the passband. If these undesired signals are not significantly attenuated then they can exceed the dynamic range of the receiver and generate IMD and HD throughout the receiver components.
A number of techniques have been proposed to avoid the SNR degradation caused by IMD and HD. For example, one approach is to attenuate undesired signal components in a received signal before passing the received signal through a non-linear receiver component. This includes increasing the performance of the input bandpass filter, or supplementing it with some tunable notch filters.
Another approach is to improve the linearity of all receiver components. This typically includes designing the components to handle larger power levels, which corresponds to increasing the dynamic range.
Yet another approach is to estimate the IMD and HD and subtract it from the received signal. This requires estimating the IMD and HD that falls into the passband. The estimation may be performed using non-linear RF/analog components along with digital signal processing (DSP). See, e.g., E. A. Keehr, et al., “A Rail-To-Rail Input Receiver Employing Successive Regeneration and Adaptive Cancellation of Intermodulation Products,” IEEE Radio Frequency integrated Circuits Symp., pp. 47-50, June 2010, and M. Valkama, et al., “Advanced Digital Signal Processing Techniques for Compensation of Nonlinear Distortion in Wideband Multicarrier Radio Receivers,” IEEE Trans. Microwave Theory Tech, vol. 54, no. 6, pp. 2356-2366, June 2006.
These techniques may also be useful when linearity requirements are already met. However, it is desirable to relax the duplexer filtering requirements, as increasing the performance of the input bandpass filter may result in a filter that has a larger size and significantly higher cost. Large duplex filters are undesirable, especially in multiple antenna architectures. Supplementing the duplex filter with one or more tunable notch filters typically results in a higher insertion loss, which is undesirable before the signal passes through the low noise amplifier (LNA).
Increasing the dynamic range of all receiver components can increases the power consumption, size and/or cost of the receiver.
The techniques that estimate the IMD and HD can significantly increase the DSP requirements of a receiver. The accuracies of the estimates depend on the interference environment. These techniques generally require additional analog to digital converters (ADCs) and RF/analog components, which can increase cost, size and/or power consumption.
These existing solutions become more problematic for wideband receivers. In a wideband receiver, large interferers may be present within channels that are intentionally passed by the RF bandpass filter, even if the portions of those channels in which the interferers appear are not used by the radio in a specific configuration.